Laravel `TokenGuard#validate` always returns false

I'm trying to implement token authentication for a Laravel project. I have the registration, logout, and middleware working, but I'm unable to validate user credentials to login. I've confirmed the user exists in the database, but Auth::guard('api')->validate($credentials) never succeeds.

config/auth.php

<?php

return [
    'defaults' => [
        'guard' => 'web',
        'passwords' => 'users',
    ],

    'guards' => [
        'web' => [
            'driver' => 'session',
            'provider' => 'users',
        ],

        'api' => [
            'driver' => 'token',
            'provider' => 'appUsers',
        ],
    ],

    'providers' => [
        'users' => [
            'driver' => 'eloquent',
            'model' => App\Models\User::class,
        ],

        'appUsers' => [
            'driver' => 'eloquent',
            'model' => App\Models\AppUser::class
        ],
    ],

    'passwords' => [
        'users' => [
            'provider' => 'users',
            'table' => 'password_resets',
            'expire' => 60,
        ],
    ],
];

app/Http/Controllers/Auth/LoginController.php

<?php

namespace App\Http\Controllers\Auth;

use App\Models\AppUser;
use App\Http\Controllers\Controller;
use Illuminate\Foundation\Auth\AuthenticatesUsers;
use Auth;
use Illuminate\Http\Request;
use Log;

class LoginController extends Controller
{
    use AuthenticatesUsers;

    public function __construct()
    {
        $this->middleware('guest', ['except' => 'logout']);
    }

    protected function guard()
    {
        return Auth::guard('api');
    }

    public function getUser(Request $request)
    {
        return response()->json([
            'success' => true,
            'body' => $this->guard()->user()
        ]);
    }

    public function postLogout()
    {
        $user = $this->guard()->user();
        if($user) {
            $user->api_token = null;
            $user->save();
        }
        return response()->json([
            'success' => true
        ]);
    }

    public function postLogin(Request $request)
    {
        $credentials = $request->only('email', 'password');
        $authenticated = $this->guard()->validate($credentials);

        if(!$authenticated) {
            return response('Invalid credentials', 401);
        }

        $user = AppUser::where('email', $request->email)->first();
        $user->api_token = str_random(60);
        $user->save();

        return response()->json([
            'success' => true,
            'body' => $user
        ]);
    }
}

Routes/api.php

Route::get('appUser', 'Auth\LoginController@getUser');
Route::post('login', 'Auth\LoginController@postLogin');
Route::post('logout', 'Auth\LoginController@postLogout')->middleware('userAuth');
Route::post('register', 'Auth\RegisterController@postRegister');

via SimpleJ